Mac OS X “.dmg” File and Command Line Tutorial

Disk image files are different than compressed files. Disk image is roughly a copy of the actual structure of disk. The OS will treat disk images just like a storage device, but a virtual one. For example, you can create a zip version of all files on a CD, but that zip file can't be used by the OS to boot the machine. With disk image file, it can. With disk image files, you can also create a image file of a CD, and that can be used to create a CD that is identical to the original CD.

Mac OS X's disk image file format is called “.dmg” files. For applications or executable files, it's better to use disk image format, because that way, all resource fork issues are done correctly, among other reasons.

To create one, use hdiutil, like this:

hdiutil create -megabytes 5 -fs HFS+ -volname myDisk ~/Desktop/myDisk

This will create a disk image with 5 mega bytes, using HFS+ format (OS X standard), and the name of the disk is “myDisk”, and the disk image file created is “myDisk.dmg”.

hdiutil man page

You can just use the “open” command. Like this: open myDisk.dmg.

The disk image will show up at 〔/Volumes/〕.

You can also use “hdid”, like this: hdid mydisk.dmg. If it is encrypted, a password GUI dialog will pop up.

To attach a encrypted dmg file without having it pop up a GUI dialog, do hdid -stdinpass mydisk.dmg.

hdid man page

To eject, first find out what disk device it is associated to by typing “df”. Here's a sample output:

/dev/disk1s2             31420     27428      3992  88% /Volumes/financial

then, use hdiutil to eject it: e.g. hdiutil detach /dev/disk2s2.

df man page

Type sudo /usr/bin/sshd. You may need to generate a passphrase if this is run for the first time. Simply go to System Prefences, Sharing, and check mark Remote Login, will automatically generate the passphrase.

Type sudo smbd restart or sudo nmbd restart.

Use “open”. e.g. open /Applications/iTunes.app. Note that the “open” command behaves as if doubling clicking on finder. In particular, if a application is already running, it will just switch to it, not launch a new process.

open man page

Sometimes, you want to launch a separate process of Carbon Emacs or Aquamacs, and want to pass it command line arguments. You can do like this: nohup /Applications/Emacs.app/Contents/MacOS/Emacs -q &.

Note, however, Terminal has a feature such that if you close the window, it'll quit all command launched from it, even if you have used “nohub”. The “nohub” will only prevent the app from quitting if you exit the shell by 【Ctrl+d】.

open -a safari myUrlStr. This will open the url in a currently running Safari browser if one has been launched. To specify other browsers, use “firefox” or “icab” for example.

In Terminal, type:

defaults write com.apple.finder AppleShowAllFiles TRUE
killall Finder

This will set a config and restart Finder.

defaults write -g com.apple.mouse.scaling

The above will show your current scaling value. To make it faster, do:

defaults read -g com.apple.mouse.scaling 5

You need to relog for this to take effect.

Use applescript, like this: osascript -e 'tell application "System Events" to sleep'

/System/Library/Frameworks/ScreenSaver.framework/Versions/A/Resources/ScreenSaverEngine.app/Contents/MacOS/ScreenSaverEngine &. You can make a bash shell alias, like this: “alias ssaver="…";” and put in 〔~/.bash_profile〕.

You can also run the screen saver on your Desktop's background. Give the command the “-background” argument.

/usr/sbin/screencapture.

screencapture man page

You can use the command line to set a preference, then you need to relaunch Dock. Type the following code in Terminal.

defaults write com.apple.dashboard mcx-disabled -boolean YES
killall Dock